The present invention is directed to a die system with a removable insert for use in the extrusion of polymers. The removable insert permits cleaning and maintenance of the die without shutting down the extrusion line.
Various mechanisms are known to facilitate the cleaning and maintenance of dies used for the extrusion of polymer materials, while minimizing down time. Molten polymers are extruded through dies to form films, strands, nonwoven webs, and other finished polymer forms. As polymer exits the die, some of the polymer clings to the die openings or xe2x80x9clips,xe2x80x9d accumulating on the exterior surface of the die. This die lip build-up gradually increases until it accumulates to a point where it breaks off, possibly causing a defect in the product. Considerable engineering goes into the design of dies to minimize this build-up. Diverging, converging, radiused, and angled die lip geometries all are examples of methods developed to minimize this build-up. However, no die design completely eliminates it. It is common practice to temporarily halt the extrusion operation to perform maintenance on the die to remove this build-up.
U.S. Pat. No. 5,435,708, issued to Kaun, discloses a melt blowing die head with opposing die lips mounted on lip guides pivotally connected to respective setback bars. To facilitate cleaning and maintenance of the die, the lips can be swung away from the spinnerette on the respective lip guides. This allows for reduced down time during maintenance of the die, but does not permit maintenance during operation of the line.
U.S. Pat. No. 5,720,986, issued to Gohlisch et al., discloses an extrusion head having a stationary part fixed on an extrusion installation. The installation includes a plurality of extrusion cylinders. The stationary part has flow channels, each in communication with a respective extrusion cylinder and a common extrusion die. Two pivotal outer parts hinged on the stationary part are selectively swingable individually between open and closed positions. The outer parts define an extrusion orifice for the die in their closed position. When the outer parts are opened, maintenance can be performed on the die,
U.S. Pat. No. 4,413,973, issued to Peters, discloses a die which has a removable extrusion plate covering the die head. The plate can be removed and replaced to minimize down time required for maintenance.
A common feature of the prior art devices is that routine cleaning and maintenance requires at least some down time. While efforts have been made to simplify maintenance and reduce down time, no device has allowed maintenance to be performed while the extrusion operation is ongoing.
The present invention is directed to an extrusion die system which permits external cleaning and maintenance to be performed without interrupting the extrusion operation. A die is provided with an opening section for the extrusion of polymer. A removable insert is provided on the exterior surface of the die lip where build-up occurs, on one or both sides of the die opening section. The inserts are located close enough to the die opening section to encompass all or substantially all of the region where the exterior build-up occurs. However, the inserts do not extend to the die opening section or over the die opening section. Instead, there is a space between the die lip or opening section and the insert or inserts.
Because the one or more removable inserts do not interfere with the die opening section, the extrusion line can continue operating while the insert is being removed and replaced. Because much of the routine external maintenance of dies is directed to removing burnt polymer from the regions surrounding the die lip, the elimination of down time during the maintenance step substantially reduces the total down time needed to maintain the die. For best results, the insert or inserts should have one end positioned as close as possible to the die lip or opening without interfering with the opening.
The one or more removable inserts can be designed into any type of polymer extrusion die including, without limitation, film, extrusion dies strand extrusion dies, melt spinning and melt blowing dies used to make nonwoven webs, and dies used to make tubes or parsons. The die openings may be straight as in a cast film die, or curved, as in a blown film die. The die may also include a plurality of smaller openings arranged in a line, circle or pattern, such as in a melt spinning or melt blowing die. Regardless of the die type, the removable insert or inserts should be positioned as close as possible to the die opening or openings, without interfering with them. In other words, the portion or portions of the die defining the openings do not form part of the removable insert. The inserts should also extend far enough away from the die openings to cover the areas where burnt polymer accumulates.
With the foregoing in mind, it is a feature and advantage of the invention to provide an extrusion die having one or more removable inserts which can be removed and replaced during operation of the die.
It is also a feature and advantage of the invention to provide a method of cleaning the exterior of an extrusion die without stopping the flow of polymer through the die.
The foregoing and other features and advantages will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are intended to be illustrative rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.
As used herein, xe2x80x9ccast film extrusionxe2x80x9d refers to the extrusion of one or more polymer layers into a flat rectangular film using a die having a straight rectangular opening.
The term xe2x80x9cblown film coextrusionxe2x80x9d refers to the extrusion of one or more polymer layers into a film having a continuous cylindrical cross-section, using a die having a cylindrical opening.
The extrusion of xe2x80x9cspunbonded fibersxe2x80x9d means the production of small diameter fibers by extending molten thermoplastic material as filaments from a plurality of fine, usually circular capillaries of a spinnerette with the diameter of the extruded filaments then being rapidly reduced as by, for example, in U.S. Pat. No. 4,340,563 to Appel et al., U.S. Pat. No. 3,692,618 to Dorschner et al., U.S. Pat. No. 3,802,817 to Matsuki et al., U.S. Pat. Nos. 3,338,992 and 3,341,394 to Kinney, U.S. Pat. No. 3,502,763 to Hartmann, U.S. Pat. No. 3,502,538 to Levy and U.S. Pat. No. 3,542,615 to Dobo et al. Spunbond fibers are quenched and generally not tacky when they are deposited onto a colleting surface. Spunbond fibers are generally continuous and have average diameters larger than 7 microns, more particularly, between about 10 and 20 microns.
The extrusion of xe2x80x9cmeltblown fibersxe2x80x9d means the production of small diameter fibers by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging high velocity gas (e.g., air) streams which attenuate the filaments of molten thermoplastic material to reduce their diameter, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly laid meltblown fibers. Such a process is disclosed, for example, in U.S. Pat. No. 3,849,241 to Butin. Meltblown fibers are microfibers which may be continuous or discontinuous, are generally smaller than 10 microns in average diameter, and are generally tacky when deposited onto a collecting surface.
The term xe2x80x9cpolymerxe2x80x9d generally includes but is not limited to, homopolymers, copolymers, such as for example, block graft, random and alternating copolymers, terpolymers, etc., and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term xe2x80x9cpolymerxe2x80x9d shall include all possible geometrical configurations of the material. These configurations include, but are not limited to isotactic, syndiotactic, atactic and random symmetries.
The term xe2x80x9cdie opening sectionxe2x80x9d refers to that portion of an extrusion die through which polymer flows. In a film extrusion die, the die opening section is generally a single opening bounded on both sides by die lips. In a filament die used to make spunbond or meltblown filaments, the die opening section is the entire portion of the die which contains small filament openings in rows or another pattern, and is bounded on both sides by the outermost rows of filament openings.